Quantum theory and the applied tests done with it. Researchers in China were able to cause an electron bound to another 1000km away to wiggle at the exact same time as they wiggled the one they had. Makes your head explode with the idea that the very concept of physical distance may not really exist.
Things that border on reality - the paranormal, unexplained phenomenon, endless philosophy on the elements that life is comprised of, such as spirit and/or soul. These things generally fascinate me the most.
Of course there's also nature, the natural order of things, the vastness of the universe, the list goes on and on..
Megalthic art, Hadrians wall, the Pyramids, stone circles like Callanish and Stonhenge, and places like New Grange. The redwood trees, the Old Man Of Hoy, the Uffington White Horse, the Nazca lines....oh I could go on and on. Nature facinates me, but I'm going to end this reply right here...lol